Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Additional Paid-in Capital	Accumulated other comprehensive income, net of deferred income tax	Retained Earnings	Treasury Shares	Ordinary Shares A	Ordinary Shares A Treasury Shares	Ordinary Shares B
Number at Dec. 31, 2014					3,064,815	16,331,577		12,061,370
Ordinary shares issued under share incentive plans						121,812
A ordinary shares purchased							11,895
Ordinary shares issued to directors						36,321
Elimination of shares indirectly owned by subsidiary							34,085
B Ordinary shares converted to A ordinary shares						7,928,004		(7,928,004)
Ordinary shares redeemed						(8,260,870)
Ordinary shares issued in connection with American Reliable acquisition						267,702
Number at Dec. 31, 2015					3,110,795	16,424,546		4,133,366
Balance at Dec. 31, 2014		$ 519,590	$ 23,384	$ 466,717	$ (101,404)	$ 2		$ 1
A ordinary shares purchased, at cost							$ (333)
Other comprehensive income (loss):
Change in unrealized holding gains (losses)			(19,436)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(10)
Unrealized foreign currency translation gains	$ 140		140
Other comprehensive income (loss), net of tax	(19,306)		(19,306)
Ordinary shares redeemed				(189,770)
Elimination of shares indirectly owned by subsidiary							$ (706)
Share compensation plans		10,272
Net income (loss)	41,469			41,469
Tax benefit on share-based compensation expense	50	10
Balance at Dec. 31, 2015	749,926	529,872	4,078	318,416	$ (102,443)	$ 2		$ 1
Ordinary shares issued under share incentive plans						115,711
A ordinary shares purchased							28,099
Ordinary shares issued to directors						35,185
Reduction in treasury shares due to redomestication					(3,138,894)	(3,138,894)
Number at Dec. 31, 2016						13,436,548		4,133,366
A ordinary shares purchased, at cost							$ (805)
Other comprehensive income (loss):
Change in unrealized holding gains (losses)			(4,751)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(3)
Unrealized foreign currency translation gains	58		58
Other comprehensive income (loss), net of tax	(4,696)		(4,696)
Reduction in treasury shares due to redomestication		(103,248)			$ 103,248	$ (1)
Share compensation plans		3,532
Net income (loss)	49,868			49,868
Tax benefit on share-based compensation expense	100	127
Balance at Dec. 31, 2016	797,951	430,283	(618)	368,284		$ 1		$ 1
Ordinary shares issued under share incentive plans						2,204
A ordinary shares purchased							29,551
Ordinary shares issued to directors						27,121
Ordinary shares redeemed						(3,397,031)
Adjustment for shares redeemed indirectly owned by subsidiary						34,085
Number at Dec. 31, 2017					29,551	10,102,927		4,133,366
A ordinary shares purchased, at cost							$ (1,159)
Other comprehensive income (loss):
Change in unrealized holding gains (losses)			8,829
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(3)
Unrealized foreign currency translation gains	775		775
Other comprehensive income (loss), net of tax	9,601		9,601
Ordinary shares redeemed				(83,015)
Adjustment for gain on shares redeemed indirectly owned by subsidiary		706		120
Share compensation plans		3,741
Net income (loss)	(9,551)			(9,551)
Balance at Dec. 31, 2017	$ 718,394	$ 434,730	$ 8,983	$ 275,838	$ (1,159)	$ 1		$ 1